Acceleration threshold sensor

ABSTRACT

An acceleration threshold sensor includes a supporting device, a seismic mass, and a connecting device with the aid of which the seismic mass is attached to the supporting device. The connecting device is provided with a predetermined breaking point interrupting the connection between the seismic mass and the supporting device when said seismic mass is subjected to an acceleration exceeding a predetermined acceleration. This sensor has a simple structural design, can be produced at a reasonable price and is always ready to carry out a measurement. Additionally, this sensor is able to store, without any auxiliary power, that it has been subjected to an acceleration exceeding a predetermined acceleration.

FIELD OF THE INVENTION

The present invention refers to an acceleration threshold oracceleration limit value sensor and to a method of producing anacceleration threshold or acceleration limit value sensor.

1. Description of Background Art

According to the background art, it is known to use macroscopicmechanical arrangements for supervising acceleration and shock events,e.g. during the transport of valuable, sensitive goods. Such anarrangement comprises a transparent plastic container with a metal ballheld by four springs at a position of rest. When a mechanicalacceleration or a shock load above a limit value occurs, the ball leavesits position of rest. Subsequently, it can be found out by opticalinspection whether such a known limit value sensor has been subjected toan acceleration or shock load above a limit value. Such macroscopicmechanical arrangements are complicated. In addition, such knownarrangements can only be read optically, i.e. by observation.

In the field of technology, numerous applications of micromechanicalstructures for detecting accelerations, speeds and forces areadditionally known. By means of such micromechanical structures, e.g.acceleration detections can be carried out by means of capacitivemeasurements as well as other measurement principles, e.g. by closingcontacts. Such micromechanical structures are, however, not able tostore information on the detected accelerations mechanically andtherefore without auxiliary power.

2. Description of Prior Art

DE 748 408 C shows a maximum acceleration meter in the case of which amaximum acceleration is related to a permanent deformation or to thebreaking of a structural material. For this purpose, rods, which areformed of a brittle or permanently deformable structural material, areconnected to a base plate on one side thereof, whereas the non-fixedends of the rods have masses attached thereto. The cross-section of sucha rod having a mass attached thereto, which is subjected to the higheststress, and, consequently, the predetermined breaking point is therespective point where the rod is fixed in the base plate. It followsthat, a maximum acceleration to be determined can be detected accordingto DE 748 408 C on the basis of a breaking of said rods.

WO-A-9111722 describes a semiconductor acceleration sensor consisting ofa fastening section, an etched silicon spring and a mass attached tosaid spring. A resistance loop is provided over the length of thespring, said resistance loop constituting a spring break indicator. Thisspring break indicator serves to indicate the readiness for operation ofthe acceleration sensor disclosed in WO-A-9111722 or a damaged conditionof said acceleration sensor. WO-A-9111722 does not disclose anacceleration threshold sensor in which the breaking of a predeterminedbreaking point is intended to indicate the occurrence of an accelerationexceeding a predetermined acceleration.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an accelerationthreshold sensor which has a simple structural design, which can beproduced at a reasonable price and is always ready to carry out ameasurement, and which is also able to store, without any auxiliarypower, that the sensor has been subjected to an acceleration exceeding apredetermined acceleration.

This object is achieved by an acceleration threshold sensor sensorcomprising a supporting device; a seismic mass; and a connecting devicewith the aid of which the seismic mass is attached to the supportingdevice, said connecting device being provided with a predeterminedbreaking point interrupting the connection between the seismic mass andthe supporting device when said seismic mass is subjected to anacceleration exceeding a predetermined acceleration, wherein thesupporting device, the seismic mass and the connecting device are formedintegrally from a silicon layer by means of a micromechanical method insuch a way that the silicon layer is provided with an opening extendingtherethrough and having arranged therein the seismic mass, the border ofsaid opening defining the supporting device, the seismic mass beingconnected on a first side thereof to the neighbouring side of thesupporting device by a first bar-shaped connection piece which definesthe predetermined breaking point, and said seismic mass being connectedto the side of the supporting device located opposite said neighbouringside by means of a second and a third connection piece, said second andthird connection pieces being connected to the seismic mass in the areaof the first side of said seismic mass and extending symmetrically onboth sides of the seismic mass to the opposite side of the supportingdevice, said second and third connection pieces being implemented suchthat they do not substantially influence the mechanical properties ofthe seismic mass.

In the case of the acceleration threshold sensor according to thepresent invention, the support means, the seismic mass and theconnection means are formed integrally from a semiconductor layer bymeans of a micromechanical method in such a way that the predeterminedbreaking point of the connection means is formed by a configuration ofthe semiconductor layer. Such an acceleration threshold sensor has asimple structural design, it can be produced at a reasonable price, itis always ready to carry out a measurement and it does not, inprinciple, require any auxiliary power for functioning. In specialcircumstances, the sensor according to the present invention can be usedin an advantageous manner in battery-operated systems in long-termoperation, where it will consume a minimum amount of energy, and it canbe read electrically at an arbitrary time.

The micromechanical acceleration threshold sensor is provided withbreaking structures consisting of a seismic mass in the form of a boardor a similar structure and of a substrate and bars which are secured tothe mass. These breaking structures can define a conductor loop which isinterrupted when the bars break. Hence, the threshold sensors accordingto the present invention can be read electrically or also optically,e.g. by visual inspection. Such breaking structures can be used assensors for limit accelerations of all kind, as detection means forspecific stress events of devices and, when several such breakingstructures are arranged in the form of an array in a system, they can beused as digital acceleration sensors.

A further object of the present invention is to provide a method ofproducing an acceleration threshold sensor.

This object is achieved by a method of producing an accelerationthreshold sensor including a supporting device, a seismic mass, and aconnecting device with the aid of which the seismic mass is attached tothe supporting device, said connecting device being provided with apredetermined breaking point interrupting the connection between theseismic mass and the supporting device when said seismic mass issubjected to an acceleration exceeding a predetermined acceleration,said method comprising the following steps or acts: providing a SIMOXsubstrate or another SOI substrate or a silicon substrate provided witha silicon oxide layer or a silicon start layer; depositing a siliconepitaxial layer or a silicon CVD layer on a thin silicon film of theSIMOX substrate or on an oxide layer of the SOI substrate or on thesilicon start layer; producing a conducting-path system and a contactingsystem on the silicon layer deposited in the previous step; producingand structuring a passivation layer over the silicon layer deposited inthe second step and the conducting-path system and the contactingsystem, with the property that this layer acts as an etching mask duringa subsequent trench-etching process; producing a back mask and carryingout a back-etching process, an oxide layer of the SIMOX substrate, anoxide layer of the SOI substrate or an oxide layer provided on thesilicon start layer acting as an etch stop; carrying out etching fromthe back of the structure for removing the oxide layer; applying aprotective layer to and structuring same on the front of the structureand applying a protective layer to the back of the structure; carryingout a subsequent trench-etching process so as to form and expose theseismic mass and the connection means of the acceleration thresholdsensor; and removing the protective layers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the drawings enclosed, in which:

FIG. 1 shows a schematic, perspective view of a first embodiment of anacceleration threshold sensor according to the present invention;

FIG. 2 shows a schematic top view of the acceleration threshold sensoraccording to FIG. 1;

FIG. 3 shows a schematic, perspective view of a second embodiment of anacceleration threshold sensor according to the present invention;

FIG. 4 shows a schematic top view of the acceleration threshold sensoraccording to FIG. 3;

FIG. 5 shows a schematic sectional view of the second embodiment of theacceleration threshold sensor along line A—A of FIG. 4; and

FIG. 6 shows a rough, schematic sectional view of an embodiment of anacceleration threshold sensor according to the present invention.

FIG. 7 shows method steps of producing an acceleration threshold sensor.

DETAILED DESCRIPTION

By means of a micromechanical acceleration threshold sensor according tothe present invention, it can be detected when limit accelerations orshocks are exceeded, and the occurrence of such limit accelerations orshocks can be stored so that, e.g. in the case of a valuable, sensitivedevice, such stress events can be proved. This is possible due to thefact that the acceleration threshold sensor according to the presentinvention is provided with a breaking structure including apredetermined breaking point which breaks in response to a specificacceleration. In the case of preferred embodiments, the predeterminedbreaking point is part of a current loop, said current loop beinginterrupted when the predetermined breaking point breaks. Thisinterruption of the current loop can be detected later on by applying avoltage. Alternatively, the breaking can be decteted by visualobservation. If an array of breaking structures for variousaccelerations is used, information about the magnitude of theacceleration load can be obtained.

The present invention is based on the fact that the breaking structuredeforms in response to an acceleration load, whereby a mechanical stressbuilds up, the bar or the bars, which act as a connecting means betweenthe seismic mass and a support, breaking at a predetermined breakingpoint when this mechanical stress exceeds a predetermined known value.The breaking of the bars interrupts e.g. a conductor loop and this hasthe effect that, when a voltage is applied later on, no current willflow. This indicates that the structure has been subjected to anacceleration load which exceeded a specific limit value. Alternatively,the structure can, however, also be read optically by inspection or bymeans of a slightly magnifying optical device.

According to preferred embodiments of the present invention, theacceleration threshold sensors are manufactured as microchips making useof silicon technology. The wafers used for this purpose are e.g. SIMOXwafers or other SOI wafers.

FIGS. 1 and 2 show a first embodiment of an acceleration thresholdsensor produced according to the present invention. The accelerationthreshold sensor comprises a seismic mass 10 connected to a supportmeans, designated generally by 14, via two bars 12 acting as aconnection means. In accordance with the embodiment shown, the seismicmass 10, the bars 12 and the support means 14 are formed integrally froma planar silicon layer. In the area of the support means 14, the siliconlayer is provided on a silicon oxide layer 16. The silicon oxide layer16, is, in turn, provided on a silicon substrate 18.

The bars 12 and, consequently, the predetermined breaking point of thisembodiment of an acceleration threshold sensor according to the presentinvention are implemented such that they are sensitive to accelerationsat right angles to the plane of the chip, i.e. accelerations in thedirection of the arrow 19 in FIG. 1. This means that the predeterminedbreaking points will break, when the seismic mass 10 is subjected to byan acceleration in the direction of the arrow 19 which exceeds apredetermined acceleration.

FIG. 2 shows a schematic top view of the acceleration threshold sensorshown in FIG. 1, the components shown in said FIG. 2 being only theseismic mass 10, the bars 12 and the part of the support means 14consisting of the silicon layer. The broken line 20 identifies the partof the silicon layer which “breaks off” when the acceleration thresholdsensor is subjected to an acceleration exceeding the predeterminedacceleration value. FIG. 2 additionally shows a conducting path 22 whichis applied to the silicon layer. This conducting path 22 is providedwith contacts 24 a and 24 b by means of which it can be detected, by theapplication of a voltage, when the acceleration threshold sensor hasbeen subjected to by an acceleration exceeding the limit acceleration,i.e. when the part outlined by the broken line 20 has “broken off”. Inthis case, the closed current path defined by the conducting path 22 isinterrupted.

In FIGS. 3 and 4, an embodiment of an acceleration threshold sensoraccording to the present invention is shown. This sensor is implementedsuch that it is sensitive to accelerations in the chip surface, i.e.accelerations in the direction of the arrow 30 in FIG. 3. Also in thecase of this embodiment, a seismic mass 32, a connection means, which isdefined by a bar 34, and a support means 36 are formed integrally from asilicon layer. The part of the silicon layer 36 forming the supportmeans is provided on an oxide layer 38 which is provided on a siliconsubstrate 40.

When the seismic mass 32 is subjected to an acceleration exceeding apredetermined acceleration in the direction of the arrow 30, the bar 34,which defines the connection means between the seismic mass 32 and thesupport means 36, will break. As can be seen in FIG. 4, a conductingpath 41 can again be applied to the silicon layer. This conducting pathcan be dispensed with, when the silicon layer itself is doped in such away that it is current carrying. As can additionally be seen in FIGS. 3and 4, thin connections 42 a, 42 b, which are formed from the siliconlayer, are attached to the seismic mass 32. These connections 42 a and42 b establish a further connection to the part of the silicon layerdefining the support means. The connections 42 a and 42 b, which act asa current supply, are geometrically designed in such a way that themechanical properties of the system are not substantially influencedthereby.

In FIG. 4, the part of the sensor device which will “break off” when theacceleration threshold sensor is subjected to an acceleration exceedinga predetermined acceleration is outlined by broken lines 44. Theconducting path is formed over the support means, the bar 34 and theconnections 42 a and 42 b in such a way that the current path isinterrupted when the acceleration threshold sensor has been subjected toan excessive acceleration. This can be detected electrically by applyinga voltage to two of the contacts 46 a, 46 b and 46 c.

FIG. 5 shows a schematic sectional view of the acceleration thresholdsensor shown in FIGS. 3 and 4 along the line A—A in FIG. 4. From thisrepresentation, it can be seen how the acceleration threshold sensor hasbeen produced from a system consisting of a silicon layer, an oxidelayer 38 and a silicon substrate 40. For producing the accelerationthreshold sensor, the silicon layer is first structured such that thestructure shown in FIG. 3 is obtained. Subsequently, the siliconsubstrate 40 and the silicon oxide layer 38 are etched away below thebreaking structure which “breaks off” in response to excessiveacceleration. The etching of the silicon layer, silicon substrate andoxide layer can also take place in a different sequence. The finishedstructure is then enclosed by means of covers 50 and 52. The cover 52 isattached to the bottom side of the silicon substrate 40. The cover 50 isattached via connection means 54 a and 54 b to the silicon layer inwhich the sensor structure is formed. Such covers protect the sensorstructure against contamination, destructions or other impairments.

In the following, suitable methods of producing an accelerationthreshold sensor according to the present invention will be discussed,partly with reference to FIG. 6. FIG. 6 is a schematic cross-sectionalview, which roughly shows the components of a sensor according to thepresent invention. In a board or wafer 100, a breaking structure 102 isformed. On one or, alternatively, on both sides of the wafer 100,support layers 104 and 106, respectively, are provided. The supportlayers have been removed in the area of the breaking structure 102. Thesupport layers 104 and 106 have attached thereto covers 108 and 110protecting the breaking structure 102, which is formed in the wafer 100,from both exposed sides. It is apparent that, when one of the covers orboth covers 108 and 110 are not provided, one of the support layers 104or 106 can be dispensed with. The support layer 104 or 106 can also bedispensed with when the covers 108 and 110 are implemented in a suitablemanner.

The breaking structure of the acceleration threshold sensor according tothe present invention is structured in a suitable material, which, inthe case of future electric reading, is preferably conductive and whichis provided in the form of a thin layer or board. The structuring iscarried out e.g. by simple mechanical methods, such as punching, cuttingor sawing, laser separation processes, lithographic processes, etchingprocesses or the like. The breaking structure can be fixedly connectedto one of the support layers 104, 106 during the structuring, thesupport layer being either a non-conductive layer or an insulator (notshown) being arranged between the support layer and the board in whichthe breaking structure is formed. The support and the possibly existinginsulating layer below the breaking structure 102 are removed below thebreaking structure. Alternatively, the breaking structure can also beconnected to a support layer after the structuring by means of joiningtechniques, e.g. by means of glueing or by anodic bonding or otherjoining methods, the support layer being again a non-conductive layer oran insulator (not shown) being arranged between the support layer andthe breaking structure. A support layer can also be provided on theother side of the board 100 in the same way. These support layers 104and 106 can have attached thereto covers 108 and 110 as a protectionagainst contaminations, destructions or other impairments of thebreaking structure 102. When the covers 108 and 110 are implemented in asuitable way, the support layers can be dispensed with.

FIG. 7 shows a method of producing an acceleration threshold sensorincluding a supporting device, a seismic mass, and a connecting devicewith the aid of which the seismic mass is attached to the supportingdevice, said connecting device being provided with a predeterminedbreaking point interrupting the connection between the seismic mass andthe supporting device when said seismic mass is subjected to anacceleration exceeding a predetermined acceleration. The method shown inFIG. 7 comprises the following steps or acts: providing a substrate(111); depositing a silicon layer on a thin silicon film of thesubstrate (112); producing a conducting-path system and a contactingsystem on the silicon layer deposited in 112 (113); producing atrench-etching mask on the deposited silicon layer and theconducting-path system and the contacting system (114); producing a backmask and carrying out a back-etching process using an oxide layer as anetch stop (115); carrying out etching from the back of the structure forremoving the oxide layer (116); applying a protective layer to andstructuring same on the front of the structure and applying a protectivelayer to the back of the structure (117); carrying out a trench-etchingprocess so as to form and expose the acceleration threshold sensorstructures (118); and removing the protective layers (119).

In the following a method of producing an electrically readable breakingstructure will be explained in detail. On the thin upper silicon film ofa SIMOX wafer, an additional silicon layer having a suitable thicknessfor adjusting the acceleration limit value is applied by epitaxy.Following this, the conducting-path and contact system with theconductor loop are produced. The conductor loop can be formed by analuminium metallization or by doping the silicon itself.

After the application of a trench-etching mask and a back mask,anisotropic etching of the silicon is carried out from the back, and theburied oxide is removed in a wet-chemical process so as to expose thearea of the breaking structures from below. Subsequently, trench etchingis carried out, which results in structuring of the breaking structuresthemselves. These etchings can also be carried out in a differentsequence. In the case of the acceleration threshold sensors shown inFIGS. 2 and 4, the contours of the silicon layer shown result fromtrench etching.

In the following, dicing and housing of the chips with the breakingstructures is carried out; prior to the sawing or dicing, the chips areprotected by a suitable protective layer, which is, however, removedafter the sawing process. Alternatively, the chips can be provided, ascan be seen in FIG. 6, with a lower and an upper cover e.g. by anodicbonding or glueing prior to the sawing or dicing.

It follows that, in accordance with the preferred embodiment forproducing an acceleration threshold sensor according to the presentinvention, a layer sequence silicon substrate/sacrificial oxidelayer/silicon layer is first produced by processing a silicon wafer. Thesacrificial oxide layer can be produced e.g. on the basis of a SIMOXprocess or on the basis of oxide deposition. The layer of material inwhich the breaking structure is formed later on can be produced e.g. byepitaxial growth of silicon on the silicon film of an SOI wafer or bydeposition of polysilicon. Following this, the rear removal of thesubstrate and of the oxide below the breaking structure is carried outby back etching. When these process steps, which are mostly carried outwet-chemically or in the vapour of an etching agent, have beenperformed, the sensor structures are produced by plasma-technologicaltrench etching. In so doing, the SOI wafer can be covered fully orpartly with a suitable protective layer, e.g. a photoresist, from theside that is not subjected to etching so as to avoid a change of theplasma-etching process, which would be detrimental to the process, whenetching through of the layer 100 begins. This protective layer issubsequently removed by plasma-technological means. Due to the trenchetching and the removal of the protective layer, the sensor structure isrendered movable and sensitive to accelerations. The present method usesfor the trench-etching process a mask of the usual passivation layerssuch as of silicon oxide, silicon nitride or silicon oxynitride. Thislayer is deposited with sufficient thickness and structured in the usualway so that the metal contacting surfaces of the components, the bondingpads, are exposed and so that the etching mask for the trench etching issimultaneously produced in accordance with the present invention. Priorto the trench-etching process, the metal contacting surfaces are coveredby a protective layer, e.g. a photoresist, in such a way that they willnot be damaged during trench etching. The protective layer is removedafter the trench etching. The trench-etching process can also be carriedout via a photoresist mask. Following this, an upper and a lower covercan be applied preferably by anodic bonding.

By means of the above-described production method, differentmicrostructures can be produced. This method can, for example, also beused for producing acceleration sensors which are sensitive toaccelerations in the chip surface or to accelerations at right angles tothe chip surface, gyroscopes as well as other microelectromechanicalstructures.

The production method is advantageous insofar as, on the one hand, themovability of the sensor structure is achieved by a dryplasmatechnological process. Hence, this method avoids the risk of theknown adherence of the movable sensor structure and of the immovablerest of the layer 100, said adherence occurring due to cohesion andadhesion forces in connection with the removal of the etching mediumafter etching with a liquid etching medium or the vapour of the etchingmedium as an etching medium. In addition, in comparison with knownmethods, the risk of future adherence is dramatically reduced due to thefact that the silicon substrate has been removed in the area of thesensor structure 32 so that adherence cannot occur there.

It follows that the present invention provides a micromechanicalacceleration threshold sensor which is adapted to be used as a sensorfor limit accelerations of all kind, as a detector means for specificstress events of devices and, produced in the form of an array, as adigital acceleration sensor. The acceleration threshold sensor accordingto the present invention has a simple structural design, it can beproduced at a reasonable price and it is always ready to carry out ameasurement. Furthermore, the acceleration threshold sensor according tothe present invention is adapted to be read preferably electrically. Thesensor according to the present invention can therefore be usedadvantageously for supervising acceleration and shock events during thetransport of valuable, sensitive goods.

What is claimed is:
 1. A method of producing an acceleration thresholdsensor including a supporting device, a seismic mass, and a connectingdevice with the aid of which the seismic mass is attached to thesupporting device, said connecting device being provided with apredetermined breaking point interrupting the connection between theseismic mass and the supporting device when said seismic mass issubjected to an acceleration exceeding a predetermined acceleration,said method comprising the following steps: a) providing a SIMOXsubstrate or another SOI substrate or a silicon substrate provided witha silicon oxide layer or a silicon start layer; b) depositing a siliconepitaxial layer or a silicon CVD layer on a thin silicon film of theSIMOX substrate or on an oxide layer of the SOI substrate or on thesilicon start layer; c) producing a conducting-path system and acontacting system on the silicon layer deposited in step b); d)producing and structuring a passivation layer over the silicon layerdeposited in step b) and the conducting-path system and the contactingsystem, with the property that this layer acts as an etching mask duringa subsequent trench-etching process; e) producing a back mask andcarrying out a back-etching process, an oxide layer of the SIMOXsubstrate, an oxide layer of the SOI substrate or an oxide layerprovided on the silicon start layer acting as an etch stop; f) carryingout etching from the back of the structure for removing the oxide layer;and g) carrying out a subsequent trench-etching process so as to formand expose the seismic mass and the connecting device of theacceleration threshold sensor.
 2. The method according to claim 1 forproducing an acceleration threshold sensor comprising a supportingdevice; a seismic mass; and a connecting device with the aid of whichthe seismic mass is attached to the supporting device, said connectingdevice being provided with a predetermined breaking point interruptingthe connection between the seismic mass and the supporting device whensaid seismic mass is subjected to an acceleration exceeding apredetermined acceleration, wherein the supporting device, the seismicmass and the connecting device are formed integrally from a siliconlayer by means of a micromechanical method in such a way that thesilicon layer is provided with an opening extending therethrough andhaving arranged therein the seismic mass, the border of said openingdefining the supporting device, the seismic mass being connected on afirst side thereof to the neighboring side of the supporting device by afirst bar-shaped connection piece which defines the predeterminedbreaking point, and said seismic mass being connected to the side of thesupporting device located opposite said neighboring side by means of asecond and a third connection piece, said second and third connectionpieces being connected to the seismic mass in the area of the first sideof said seismic mass and extending symmetrically on both sides of theseismic mass to the opposite side of the supporting device, said secondand third connection pieces being implemented such that they do notsubstantially influence the mechanical properties of the seismic mass.3. The method according to claim 1, comprising the additional methodstep of producing a doped area in the deposited silicon epitaxial layerafter the application of said silicon epitaxial layer.
 4. The methodaccording to claim 1, wherein, after trench etching, the passivationlayer serves as a passivation of the doped areas, of the metallizationsystem and of the contacting system.
 5. The method according to claim 1,comprising the additional method step of applying a protective layer toand structuring it on the front of the structure before thetrench-etching and removing said protective layer after thetrench-etching.
 6. The method according to claim 1, comprising theadditional method step of applying a complete or a partial protectivelayer to the back before the trench etching and removing said protectivelayer after the trench etching.
 7. The method according to claim 1,comprising the additional method step of trench etching making use of aphotoresist mask.